playbin: Don't ref_sink() the sinks twice

Since we're already sunk floating reference, we shouldn't call ref_sink
again, which increases the ref_count and cause leaks.

https://bugzilla.gnome.org/show_bug.cgi?id=789547

gst/playback/gstplaybin2.c

@@ -4663,7 +4663,6 @@ autoplug_select_cb (GstElement * decodebin, GstPad * pad,
                   gst_plugin_feature_get_name (GST_PLUGIN_FEATURE (ave->sink)));
               continue;
             }
-            gst_object_ref_sink (*sinkp);
             created_sink = TRUE;
           }
         }